Ach, du lieber Herr Spencer!
Somelwhere along the way (an old pop ltune, you know), you
misinterpreted the word "Ur". It refers not to the ancient Babylonian
city you mention. Rather, if you grab an English-German dictionary,
you'll find that the German word "Ur" means "original" or "the
prototype". Therefore, the Cambridge use of the term
"Ur-Jazz" simply means the orginal form of Jazz in its development down
in good, old New Orleans and its environs.
Dare I say "Vielen Dank"? (Many thanks).
Regards, Harold Smith